Increasing sensitivity is the most effective means to improve various photographic properties of the silver halide light-sensitive photographic emulsion. For example, high sensitive light-sensitive color photographic materials of recent years were realized by making the emulsion high sensitivity. About improvement of picture quality, it is widely known in the art that it is possible to improve graininess by using a silver halide emulsion containing silver halide grains of smaller grain size and enhanced sensitivity.
Moreover, in the field of x-ray photography, in order to maintain high sensitivity while achieving a high sharpness and effectively cutting off crossover light, development of a light-sensitive silver halide emulsion with increased sensitivity is indispensable. Accordingly, various research and development have been made, attempting to produce a silver halide light-sensitive emulsion having enhanced sensitivity.
Especially in recent years, a lot of techniques of attaining high sensitivity, by using of silver halide grain of tabular shape has been reported in the art. Those examples are described, for example, in Japanese Patent O.P.I. Publications No.58-111935(1983), No.58-111936(1983), No.58-111937(1983), No.58-113927(1983) and No.59-99433(1984), etc.
Since the surface area of the tabular silver halide grain is larger than that of a so-called regular shape silver halide grain, such as cubic or octahedral grains, so that it is possible to increase the adsorption amount of sensitizing dye on the surface of the grain, and, as a result, there is an advantage that elevation of light-sensitivity may easily be achieved.
Japanese Patent O.P.I. Publication No.63-92942(1988) discloses a technique, in which a core with a high silver iodide content is provided inside the tabular silver halide grain. Moreover, Japanese Patent O.P.I. Publication No.63-151618(1988) discloses a technique, in which a tabular silver halide grain having hexagonal shape is used. And in these references effect of enhancing sensitivity is shown.
Besides these, a technique concerning distributing silver halide composition inside the tabular silver halide grain is disclosed in Japanese Patent O.P.I. Publications No.63-106746(1988),No.1-183644(1989),No.1-279237(1989), etc.
Further, in connection with the crystal structure of the tabular silver halide grain, some techniques of the shape of the grain and a parallel twin plane are also disclosed. For example, Japanese Patent O.P.I. publication No.1-131541(1989) discloses a technique to improve sensitivity and graininess by using a round tabular grain.
Japanese Patent O.P.I. Publication No.63-163451(1988), discloses a technique to use a tabular silver halide crystal having a pair of parallel twin planes, of which ratio (t/1) of distance between twin planes (1) and thickness of the grain (t) is not less than 5. And improved effects in sensitivity and graininess are exhibited. Therein, a technique for enhancing sensitivity by uniforming the distance between the twin planes among the grains and a technique, whereby to improve sensitivity and graininess is disclosed.
International Patent Application No. WO91/18320 discloses a technique of using a tabular silver halide grain of which the distance between twin planes is less than 0.012 microns: In this reference, it is described that by this technique enhancement of sensitivity was realized.
European Patent Application No. EP515894A1 refers to a technique by which to attain enhanced sensitivity by using a tabular silver halide grain of which flatness defined in terms of (grain size)/(thickness) is not less than 25 and the proportion of (111) face in edge-side surface is made less than 75%.
On the other hand, attempts to improve defects of the tabular silver halide grain have also been made. For example, Japanese Patent O.P.I. publication No.3-142439 (1991) discloses a technique to improve preservability under high humidity by employing a silver halide emulsion containing a silver halide grain of which an aspect ratio is not less than 3 and making proportion of projection area of the tabular grains having (111) face and (100) face not less than 50%.
Moreover, tabular silver halide grain has a defect that its pressure resistance characteristics are not so good.
Herein, the pressure resistance characteristics denote two photographic phenomena,that is to say,
(a) a phenomenon of so-called pressure fogging,in which when pressure is applied to a silver halide light-sensitive photographic material, an unexposed portion of the light-sensitive material comes to be developed or so-called pressure fog takes place, and PA1 (b) a phenomenon of so-called pressure desensitization in which sensitivity lowering at the time of exposure takes place. PA1 (i) a silver halide light-sensitive photographic emulsion comprising tabular silver halide grains, wherein said tabular silver halide grain is characterized in that PA1 (standard deviation of grain size)/(average of grain size).times.100 =Grain size distribution width(%), PA1 (standard deviation of thickness distribution)/(average thickness).times.100 =thickness distribution width(%),
These defects of the silver halide light-sensitive photographic material can be a serious fault and lowers its commercial value. In general, silver halide grain is sensitive to pressure and, its sensitivity against pressure increases sharply corresponding to the increase of light-sensitivity and thus, this phenomenon is remarkable in the silver halide light-sensitive photographic material in which tabular silver halide grain is used. The reason for this is considered as follows: that is to say, a larger amount of moment is applied to a tabular grain as compared with a spherical shape grain having the same volume, even if they have the same mechanical strength, and thus mechanical strength of the tabular grain as a whole becomes weaker than that of the spherical grain.
Besides the shape of the silver halide grain, pressure resistance characteristics of the silver halide crystal depend upon distribution of halide composition inside the silver halide grain and on the conditions of chemical sensitization.
In general, desensitization due to pressure is likely to cause when the degree of chemical sensitization is not enough, or, in the case of lacking in the chemical ripening. When the level of chemical sensitization is excess, desensitization due to pressure tends to decrease, however, in this case, pressure fogging is inclined to be greater. In general, when the level of chemical sensitization is excess, pressure desensitization decreases, however, in this case, pressure fogging tends to increase. Moreover, when the silver the halide grain has an internal high iodide content, there is a tendency that the pressure desensitization increases although pressure fogging is improved.
As a means to overcome deterioration of such pressure resistance characteristics, some means for improving pressure characteristics of silver halide emulsion are disclosed in, for example, Japanese Patent O.P.I. Publications No. 59-99433(1974), No. 63-30.1937(1988), No. 63-149641(1988), No. 63-106746(1988), No. 63-151618(1988), No. 63-220238(1988), Japanese Patent O.P.I. publications No. 1-131541(1989),No. 2-193138(1990), No. 3-172836(1992) and No. 3-231739(1992). However, these means have not achieved sufficient effect of improvement.